Interacting binaries on the Main Sequence as in-situ tracers of mass transfer efficiency and stability

TL;DR

This work uses massive Algol-type binaries on the Main Sequence across the Milky Way, LMC, and SMC to empirically constrain thermal-timescale mass transfer, its efficiency $\\varepsilon$, and angular momentum loss characterized by $\\gamma$. By mapping current binary properties to initial conditions via an analytical framework and validating with detailed binary-evolution grids, the authors show that neither fully conservative nor fully non-conservative mass transfer can explain all systems, and that stability must persist down to $q_{\\rm i} \sim 0.6$ in many cases. They also explore disk-mediated accretion as a mechanism to achieve higher $\\varepsilon$ and find metallicity trends, with SMC systems generally displaying less efficient mass transfer than LMC and MW counterparts. The results provide robust, in-situ constraints that inform population-synthesis predictions for stripped-envelope supernovae and gravitational-wave progenitors, while highlighting the need for improved observational constraints and multi-dimensional modelling of the mass transfer phase.

Abstract

Understanding the transfer of mass and angular momentum in binary interactions is crucial for modelling the evolution of any interacting binary after the first mass transfer phase. We constrain the efficiency and stability of thermal timescale mass transfer in massive binary evolution using the observed population of massive interacting binaries on the Main Sequence (`Algols') in the Milky Way, Large and Small Magellanic Clouds. Assuming the present-day mass of the donor star represents its initial convective core mass at Zero-Age Main Sequence, we estimate its initial mass using detailed stellar evolution models. From the initial donor mass, we calculate the range of initial accretor masses (for different mass transfer efficiencies). By imposing physical constraints on the above initial parameter ranges, we derive the mass transfer efficiency, stability and angular momentum loss that can reproduce the current properties of each Algol binary. We find that purely conservative or non-conservative mass transfer cannot explain the current mass ratio and orbital period of all massive Algols. Angular momentum conservation rules out conservative mass transfer in $\sim$28\,\% of massive Algols in the SMC. About three-quarters of all massive Algols are consistent with having undergone inefficient mass transfer ($\lesssim$\,50\,\%), while the remaining systems, mostly residing in the LMC and Milky Way, require mass transfer to have been more efficient than 25\%. The current sample of massive Algols does not require mass transfer to be efficient at the shortest orbital periods (2\,d) at any metallicity. We find evidence that mass transfer on the Main Sequence needs to be stable for initial accretor-to-donor mass ratios as unequal as $\sim 0.6$. The massive Algols in the SMC seem to have undergone less efficient mass transfer than those in the LMC and Milky Way. (Abridged)

Figures (7)

• Figure 1: The parameter space of mass transfer efficiency $\varepsilon$ (bottom panels) and initial orbital period $P_{\rm orb,i}$ (top panels) as a function of the decreasing initial mass ratio $q_{\rm i}$ for LZ Cep (left panel, representing a typical case) and O026631 (right panel, representing an outlier system). The white region indicates the most likely values of mass transfer efficiency and specific angular momentum loss from the rotation-limited accretion model. Different colours correspond to curves of $P_{\rm orb,i}$ for alternative assumptions of $\gamma$=1 (red), 2 (green) and $M_{\rm d}/M_{\rm a}$ (purple, isotropic remission from the surface of the accretor). In the right panel, the purple star shows the initial orbital period for $\varepsilon=0$ (Eq. (\ref{['eq:epsilon0']})). The black and blue hatched regions correspond to $\gamma\leq0$ and $\gamma\geq3$, respectively. The dark grey colour shows the region where all detailed binary evolution models merge. All detailed binary evolution models survive the thermal timescale mass transfer to the left of the light grey region (the highest value of $q_{\rm i}$ in the light grey region is the $q_{\rm max}$ limit). Models in the light grey region merge or survive depending on the binary orbital period; the shortest-period models merge, and vice versa (the lowest value of $q_{\rm i}$ in this region is the $q_{\rm min}$ limit). The orange dashed line shows the limiting mass transfer efficiency as a function of the initial mass ratio for which non-rotating models of Schurmann2024 undergo L2 overflow. The orange shaded region, therefore, denotes the parameter space where an accretion disk-mediated mass transfer onto the accretor may not be stable. The upper limit to the top panels is given by $P_{\rm orb,i,max}$ calculated for each system.
• Figure 2: Inferred range of mass transfer efficiency (black lines) as a function of the orbital period of massive Algols in the LMC and Milky Way (left panel) and the SMC (right panel). Triangles indicate the minimum and maximum mass transfer efficiency possible for each system, in the rotationally limited mass accretion scheme. The 2D histogram displays the calculated mass transfer efficiency in detailed binary evolution models that also incorporate the rotationally limited mass accretion prescription (note the log scale on the colour bar). The total probability is normalised such that the integrated sum over the entire area is 1.
• Figure 3: Disallowed range (black bar - initial accretor mass becomes larger than initial donor mass; grey bar - L2 overflow of the accretor) and allowed range (light blue - rotationally limited mass accretion scheme; dark blue - disk supported mass transfer scheme) of mass transfer efficiencies for each system in the LMC & Milky Way (left panel) and SMC (right panel), arranged in increasing order of observed mass ratio.
• Figure 4: 2D-histogram showing the distribution of the donor masses as a function of the logarithm of initial donor mass during the Algol phase, for the detailed binary evolution models with LMC metallicity Sen2022. The total probability is normalised such that the sum over all pixels for each logarithmic initial mass bin is unity (overplotted numbers show contribution from each pixel up to two significant digits). The red and blue curves show the convective core mass of the donor at Zero-Age Main Sequence with and without overshooting, respectively. The orange curve indicates that 80% of donor models during the Algol phase are stripped below the curve.
• Figure 5: Same as Fig. \ref{['fig:individual_examples']}, for all massive Algols in the SMC.